The present invention relates to orthokeratology, i.e. shaping of the cornea to correct for refractive errors, and more particularly to an accelerated method of reshaping the corneal tissues wherein the cornea is softened, shaped with a mold, and thereafter rapidly stabilized so that the cornea immediately retains the new shape.
Millions of people worldwide have refractive errors of the eye which cause them to seek out corrective eyeglasses and/or contact lenses. Among the most common refractive errors are myopia (inability to see distant objects), hyperopia (inability to see near objects), and astigmatism (asymmetric sloping of the cornea whereby the curvature is different in different planes). Each of the above-noted defects is usually corrected by means of corrective eyeglasses or contact lenses. Corrective eyeglasses correct refractive errors by changing the angle of light with a lens before it reaches the cornea. Contact lenses correct refractive errors by replacing the misshapen anterior curvature of the cornea with a curvature which is calculated to render the eye emmetropic. While corrective eyeglasses and contact lenses are highly effective for temporarily correcting these problems, i.e. while the glasses or contacts are in place, the physical defects of the cornea are never corrected and thus require lifetime wear of the glasses or contacts. Accordingly, there has been an ongoing search for effective methods of correcting refractive errors of the eye by physically altering the anterior curvature of the cornea so that corrective lenses are no longer required.
Among the many solutions to refractive eye problems are surgical procedures in which the cornea is surgically altered. While effective, the existing surgical modification techniques have significant risk factors and drawbacks, including human error, infection, long healing time, and temporary loss of sight during recovery. Furthermore, there are significant psychological fears associated with voluntary eye surgery. The chances of permanently damaging the eye do not usually outweigh the discomfort of wearing glasses or contacts in most cases. For obvious reasons, invasive surgical modification of the cornea has not been well received as a purely voluntary procedure.
A non-invasive technique for physically altering the anterior curvature of the cornea which has received acceptance is Laser Photorefractive Keratectomy wherein an excimer laser is used to selectively strip away (ablate) outer layers of the cornea to produce a more spherical curvature. The laser method has achieved a high degree of success. However, there are certain drawbacks to this procedure, including temporary reductions of visual acuity during healing, delayed visual recovery, pain, stromal haze, temporary hyperopia, night glare, halos, and infectious keratitis.
A lesser known non-surgical technique, orthokeratology, which forms the general basis for the present invention, involves the use of a series of progressive contact lenses that are intended to gradually reshape the cornea and produce a more spherical anterior curvature. The process usually involves the fitting of 3 to 6 pairs of contact lenses, and usually takes approximately 3-6 months to achieve optimal reshaping. The theory behind Orthokeratology is that the cornea is very pliable and can be physically reshaped over time. The thickest layer of the cornea, known as the stroma, is formed from alternating lamella of fine collagen fibrils which form a pliable matrix of tissue. While the collagen tissues are pliable, they unfortunately also exhibit shape memory, and unless retainer lenses are worn daily to maintain the desired shape, the cornea will rapidly regress to the original shape.
Additional development work in the field of orthokeratology has yielded accelerated methods of orthokeratology wherein chemical, enzymatic and/or other agents are used to soften the cornea. For example, the Neefe U.S. Pat. Nos. 3,760,807, 3,776,230 and 3,831,604 collectively describe the use of chemicals such as proparacaine hydrochloride, dyclonine hydrochloride, chlorine in solution, the application of heat to the cornea through heated molds, the application of heat in the form of ultrasonic energy, and the use of proteolytic enzymes to soften the cornea for reshaping. Furthermore, the Kelman and DeVore U.S. Pat. Nos. 4,713,446, 4,851,513, 4,969,912, 5,201,764 and 5,492,135 each describe various chemical agents for treating and/or softening both natural and artificial collagen materials for ophthalmic uses.
Of the various prior art available in this subject area, the Harris U.S. Pat. Nos. 5,270,051 and 5,626,865 are believed to be the closest prior art to the subject matter of the invention of which the applicant is aware. The Harris patents describe a method of accelerated shaping of the cornea by releasing enzymes, such as hyalurodinase, into the cornea to temporarily soften the cornea, and thereafter fitting the cornea with a rigid contact lens which has a curvature that will correct the refractive error. The softened cornea then reshapes its curvature to the curvature of the contact lens rendering the eye emmetropic. The speed of the shaping process is significantly increased by the use of the softening agent, and reduces the treatment period from months to days. After shaping, a retainer lens is worn for a period of several days while the enzyme is allowed to dissipate from the cornea, and the cornea "hardens" to retain the new emmetropic configuration.
While softening of the corneal tissues does speed in reshaping of the cornea, there has been very little success in developing a successful method of rapidly restabilizing the corneal tissues in their new configuration after reshaping. The methods as described in the Harris U.S. Pat. Nos. 5,270,051 and 5,626,865 simply allow the softening agent to dissipate over time, after which time the lens can be removed. The only prior art known to the Applicant in the context of "active" corneal restabilization, is the Neefe U.S. Pat. No. 3,760,807 which describes a method of administering oral Vitamin C as a means for speeding the hardening of the cornea after use of the corneal softening agent has been discontinued. However, speeding up the hardening of the cornea in this context means to possibly reduce the hardening time from weeks to days.
The instant invention provides improved methods of accelerated orthokeratology which focus on rapidly restabilizing the corneal tissues in their new configuration after reshaping. The successful development of a rapid method of restabilizing the corneal tissues provides the final key step in a rapid non-surgical treatment alternative for physically reshaping of the cornea. In the context of the present invention, a patient could expect to enter the doctor's office on an outpatient basis, have the entire treatment completed within hours, and leave the office with a completely and reshaped cornea and no need for further use of contacts or glasses.
Generally speaking, the improved method as described herein comprise a three step process of: 1) softening or "destabilizing" the cornea with a chemical or enzymatic softening agent; 2) applying a mold to reshape the cornea to a desired anterior curvature; and 3) rapidly restabilizing the corneal tissues with a "stabilizing agent" which is effective for immediately initiating cross-linking of the collagen matrix. The term "stabilizing agent" as used herein is intended to include both chemical agents as well as external energy, such as light energy, applied to the cornea. More specifically, the contemplated agents for restabilizing the cornea include chemical cross linking agents, ultraviolet irradiation, thermal radiation, visible light irradiation, and microwave irradiation. The preferred method of restabilizing the cornea presently comprises exposure UV light energy, in conjunction with a photoactivator or initiator. The invention further provides novel apparatus for use in the described methods.
In the preferred method an annular staging device is aligned and secured with a biological glue over the cornea for guiding delivery of the softening agents, mold and UV light to the cornea. The staging device preferably includes an annular flexible gasket on the lower rim to prevent leakage of the chemicals introduced into the staging device. A chemical softening agent, such as glutaric anhydride is introduced into the staging device to soften the cornea. Glutaric anhydride is known to destabilize cross-links between the collagen fibrils, and acts to soften the corneal tissue enough to allow shaping with minimal external pressure. After treatment with the chemical softener, a specially designed mold of predetermined curvature and configuration is fitted into the staging device. Slight downward pressure is applied to the mold for a predetermined period of time (1-10 minutes) to re-shape the cornea. The mold is thereafter maintained in position while a UV light source is positioned above the mold within the staging device. The mold is preferably fashioned from a material which is transparent and non-UV absorbing, such as clear acrylic. UV light is applied to the cornea for a predetermined time wherein the UV light cross-links, the collagen fibrils and restabilizes the corneal tissue so that the cornea immediately and retains its new shape. The stabilization step can also be used for patients having already undergone long term orthokeratology to eliminate the need to continue wearing a retainer to maintain the shape of the cornea.
Accordingly, among the objects of the instant invention are: the provision of an accelerated method of orthokeratology including a means for rapidly restabilizing the cornea tissues after reshaping; the provision of such a method wherein the cornea is softened with a softening agent which destabilizes the collagen fibrils in the cornea; the provision of such a method wherein the softened cornea is thereafter molded with a mold having a predetermined curvature and configuration; the provision of such a method wherein the cornea is stabilized by applying UV light to cross-link the collagen fibril network; the provision of apparatus for performing the method including an staging device for limiting the treatment area of the cornea and preventing leakage of treatment chemicals outside of the designated area; and the provision of such a staging device wherein the staging device guides application of the mold and light energy to the cornea.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.